Switching power supply circuits are utilized in a number of different circuit applications. There are three basic switching power supply topologies in common use; buck converter, boost converter, and buck boost converter. These topologies are generally non-isolated, that is, the input and output voltages share a common ground. There are, however, isolated derivations of these non-isolated topologies. The differing topologies refer to how the switches, output inductor and output capacitor associated therewith are interconnected. Each topology has unique properties that include the steady-state voltage conversion ratios, the nature of the input and output currents, and the character of the output voltage ripple. Another property is the frequency response of the duty cycle-to-output voltage transfer function.
A single output buck converter topology is also referred to as a buck converter, buck power stage or a step-down power stage (because the output is always less than the input). The input current for a buck power stage is said to be discontinuous or pulsating if a switching current pulses from zero or some negative value to some positive output current value every switching cycle. The output current for a buck power stage is said to be continuous or non-pulsating because the output current is supplied by an output inductor/capacitor combination. In the latter event, the inductor current never reaches a zero or negative value.
An exemplary basic buck converter circuit is illustrated in prior art FIG. 1A, and designated at reference numeral 10. When a power switch 12 is activated, the switch behaves like a closed circuit, as illustrated in prior art FIG. 1B, and the input voltage VIN is applied to an inductor 14, and power is delivered to an output load 16. The output load voltage is VOUT=VIN−VL, wherein the VL, the voltage across the inductor 14, is given by L(di/dt). The output voltage VOUT also is formed across a capacitor 18, thus the capacitor charges and the output voltage increases each time the switch 12 is closed.
When the switch 12 is deactivated, or turned off, the switch 12 behaves as an open circuit, as illustrated in prior art FIG. 1C, and the voltage across the inductor 14 reverses due to inductive flyback, thus making a circuit diode 20 forward biased. The circuit loop generated by the diode 20 allows the energy stored in the inductor 14 to be delivered to the output load 16, wherein the output current is smoothed by the capacitor 18. Typical waveforms for a buck converter are shown in FIG. 2. The power switch 12 is switched at a relatively high frequency (e.g., between about 20 KHz and about 300 KHz for most converters) to produce a chopped output voltage, however, the inductor 14 and capacitor 18 together operate as an LC filter to produce a relatively smooth output voltage having a DC component with a small ripple voltage overlying the DC value (see, e.g., output voltage waveform of FIG. 2). The ripple voltage can be controlled by varying the duty cycle of the power switch control voltage.
The base principle of operation in the above buck converter 10 is often utilized in hysteretic dc-dc converters, as illustrated in prior art FIG. 3, and designated at reference numeral 30. The circuit 30 is similar in various respects to the buck converter 10 of FIG. 1A and employs a unity gain buffer 32 serially coupled to an analog comparator circuit 34 having a hysteresis VH. The comparator 34 compares the input reference voltage VREF to the circuit output voltage VOUT and provides an output signal at node 36, which is a function of the comparison and constitutes a generally square wave. An exemplary output voltage waveform for the circuit 30 is illustrated in FIG. 4.
The hysteresis VH of the comparator 34 impacts the operation of the circuit 30 in the following manner. As the output VOUT falls below a voltage VREF−VH, the comparator 34 trips and the output thereof at node 36 goes from zero to the supply, ideally, which then is fed to the circuit output VOUT (wherein, VOUT is a function of the output of the comparator and the duty cycle of the driver). Similarly, as VOUT increases to a voltage VOUT+VH, the comparator 34 again trips and the output thereof at node 36 decreases to zero volts, which is fed to the circuit output VOUT. Therefore the hysteresis VH of the comparator 34 dictates an amount of voltage ripple (2*VH) about the target reference voltage VREF, as illustrated in FIG. 4, and, in conjunction with the output capacitor dictates a natural frequency of the ripple voltage at the output VOUT.
Single output buck converters work well in applications and/or devices that employ a single input voltage. However, some applications and/or devices utilize two power sources. For example, a digital signal processor (DSP) generally employs two power supplies; one power supply (1.8V) is to power an I/O ring and the other (1.2V) is to power a digital core. Dual voltage outputs of the power converter are also reported to reduce power dissipation. Two single output buck converters can be employed in such instances, but at a relatively high cost in terms of power utilization, area utilization, and component costs. Typically, inductors are the highest cost component and employing two buck converters requires two inductors. Additionally, more switches are then employed, which can result in greater power consumption.
Single inductor dual output converters are available, but have limited output power availability. Conventional control algorithms that are used to operate these converters result in relatively high peak-to-peak currents that increase output voltage ripple for both outputs and also increase electromagnetic interference (EMI) radiation, which reduces their suitability for wireless applications. Furthermore, the control of one output often interferes with the control of a second output, particularly at full output load currents. This interference is also referred to as cross talk.